Power-scalable nonlinear optical wavelength converter

ABSTRACT

A system includes a nonlinear crystal positioned such that a focus of a laser beam is outside the nonlinear crystal in at least one plane perpendicular to a beam propagation direction of the laser beam. The nonlinear crystal is disposed in a crystal mount assembly. A laser beam may be directed at the nonlinear crystal for wavelength conversion. The system may be used as a deep-UV wavelength converter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Pat. No. 9,841,655, filed Jun.29, 2016, which claims priority to the provisional patent applicationfiled Jul. 1, 2015 and assigned U.S. App. No. 62/187,739, thedisclosures of which are hereby incorporated by reference.

FIELD OF THE DISCLOSURE

This disclosure relates to optical wavelength conversion.

BACKGROUND OF THE DISCLOSURE

Inspection processes are used at various steps during semiconductormanufacturing to detect defects on wafers to increase yield. However, asthe dimensions of semiconductor devices decrease, inspection becomesmore important to the successful manufacture of semiconductor devicesbecause smaller defects can cause the devices to fail. Semiconductormanufacturers seek improved sensitivity to particles, anomalies, andother defect types, while maintaining overall inspection speed (inwafers per hour) in wafer inspection systems.

Each successive node of semiconductor manufacturing requires detectionof smaller defects and particles on the wafer. Therefore, higher powerand shorter wavelength UV (ultraviolet) lasers for wafer inspection areneeded. Because the defect or particle size is reduced, the fraction ofthe light reflected or scattered by that defect or particle is alsotypically reduced. As a result, an improved signal-to-noise ratio may beneeded to detect smaller defects and particles. If a brighter lightsource is used to illuminate the defect or particle, then more photonswill be scattered or reflected and the signal-to-noise ratio can beimproved if other noise sources are controlled. Using shorterwavelengths can further improve the sensitivity to smaller defectsbecause the fraction of light scattered by a particle smaller than thewavelength of light increases as the wavelength decreases.

Some inspection tools for wafers and reticle inspection used in thesemiconductor industry rely on deep-ultraviolet (DUV) radiation. Some ofthe most compact, efficient, and cost effective sources of laserradiation in the UV and DUV spectral regions are based on wavelengthconversion of solid-state laser radiation in nonlinear optical crystals.When exposed to high-power DUV-radiation, optical components, includingnonlinear optical crystals, are prone to optically induced damage, whichlimits the maximum power density present on or in each individualcomponent. This power density limitation forces the optics designer tomake trade-offs between achievable DUV-power, spatial beam quality,component lifetime, and form factor of the wavelength converter device.Optimizing the beam size in the nonlinear crystal may be needed to takeadvantage of and trade-off between harmonic (DUV) power, spatial beamquality, and nonlinear crystal lifetime. Meanwhile, a low power densityon the optics may be needed to achieve the desired component lifetime.

If one or multiple of the wavelengths involved in the nonlinearwavelength conversion process are in the DUV region, the DUV-radiationis prone to cause optically induced damage not only in the nonlinearcrystal, but also to other optical components in the beam shapingoptics. Limiting the power density both in the crystal and on/in thebeam shaping optics can become important in this case. The acceptablepower density on the beam shaping optics can be considerably lower thanin the nonlinear crystal itself. This requirement is in part due tomaterial properties (e.g., in the case of fused silica) and in part dueto the fact that the spot shifting schemes commonly used for nonlinearcrystals cannot be applied to many optics components (e.g., sphericallenses) without introducing misalignment and aberrations into the beampath. This makes it necessary to position the beam shaping optics at adistance from the nonlinear crystal where the beam has diverged enoughto reduce the optical power density to an acceptable level. Increasingthe focus size in the nonlinear crystal decreases the beam divergence.The required distance from the nonlinear crystal to the beam shapingoptics increases accordingly, so that the wavelength conversion modulemay become larger than desired.

The nonlinear crystal can be periodically shifted perpendicular to thebeam, which uses multiple crystal locations. If one area of thenonlinear crystal is damaged, then the nonlinear crystal is movedrelative to the beam so that the beam is projected onto a different,undamaged area. While this may prolong the period before the nonlinearcrystal must be replaced, this fails to address the cause of any damageto the nonlinear crystal.

Therefore, what is needed is an improved nonlinear optical wavelengthconverter.

BRIEF SUMMARY OF THE DISCLOSURE

In a first embodiment, a system is provided. A system comprises a lasersource, a nonlinear crystal, and a crystal mount assembly. The lasersource is configured to generate a laser beam. The nonlinear crystal isconfigured for wavelength conversion. The nonlinear crystal ispositioned such that a focus of the laser beam is outside the nonlinearcrystal in at least one plane perpendicular to a beam propagationdirection of the laser beam. The nonlinear crystal is disposed on thecrystal mount assembly.

Beam shaping optics can be disposed between the laser source and thenonlinear crystal and/or can be disposed downstream of the nonlinearcrystal in the beam propagation direction.

The crystal mount assembly can be configured to adjust a beam size ofthe laser beam in the nonlinear crystal by adjusting a distance betweena center of the nonlinear crystal and the focus.

The nonlinear crystal can be positioned such that the focus of the laserbeam is outside the nonlinear crystal in the at least one planeperpendicular to the beam propagation direction of the laser beam with aRayleigh range configured such that time averaged fundamental opticalpower density or harmonic optical power density at the spatial peak ofthe beam profile on or in at least one optical component in the systemis limited to below 1 MW/cm².

The laser beam can be a pulsed laser beam. The nonlinear crystal can bepositioned such that the focus of the pulsed laser beam is outside thenonlinear crystal in the at least one plane perpendicular to the beampropagation direction of the pulsed laser beam with a Rayleigh rangeconfigured such that fundamental optical fluence power density orharmonic optical fluence on or in at least one optical component of thesystem is limited to below 10 J/cm².

The crystal mount assembly can include a plurality of mounting featuresat different distances from the laser source. The crystal mount assemblycan be configured to be disposed on one of the mounting features and abeam size of the laser beam in the nonlinear crystal can be provided byselecting one of the mounting features.

Using the plurality of mounting features, the nonlinear crystal can bepositioned such that the focus of the laser beam is outside thenonlinear crystal in the at least one plane perpendicular to the beampropagation direction of the laser beam with a Rayleigh range configuredsuch that time averaged fundamental optical power density or harmonicoptical power density at the spatial peak of the beam profile on or inat least one optical component in the system is limited to below 1MW/cm².

Using the plurality of mounting features, the laser beam can be a pulsedlaser beam. The nonlinear crystal can be positioned such that the focusof the pulsed laser beam is outside the nonlinear crystal in the atleast one plane perpendicular to the beam propagation direction of thepulsed laser beam with a Rayleigh range configured such that thefundamental optical fluence or harmonic optical fluence on or in atleast one optical component of the system is limited to below 10 J/cm².

The crystal mount assembly can be adjustable. The nonlinear crystal canbe positioned such that the focus of the laser beam is outside thenonlinear crystal in the at least one plane perpendicular to the beampropagation direction of the laser beam with a Rayleigh range configuredsuch that time-averaged fundamental optical power density or harmonicoptical power density at the spatial peak of the beam profile on or inat least one optical component in the system is limited to below 1MW/cm².

The laser beam can be a pulsed laser beam and the crystal mount assemblycan be adjustable. The nonlinear crystal is positioned such that thefocus of a pulsed laser beam is outside the nonlinear crystal in the atleast one plane perpendicular to the beam propagation direction of thepulsed laser beam with a Rayleigh range configured such that fundamentaloptical fluence or harmonic optical fluence on or in at least oneoptical component in the system is limited to below 10 J/cm².

The focus can be at least one of circular, elliptical, or astigmatic.

The focus can be elliptical and the focus in a plane parallel to awalk-off is larger than the focus in a plane perpendicular to thewalk-off.

The focus can be astigmatic such that, for example, the focus in oneplane is inside the nonlinear crystal and the focus in another plane isoutside the nonlinear crystal.

The focus can be astigmatic and elliptical. The focus in one plane isinside the nonlinear crystal and the focus in another plane is outsidethe nonlinear crystal. The focus inside the nonlinear crystal has alarger width than the focus outside the nonlinear crystal.

The system can be configured such that the wavelength conversion is oneof second harmonic generation, sum-frequency generation, or differencefrequency generation.

The system can further include an adjustment assembly connected to thecrystal mount assembly. The adjustment assembly may be, for example, ascrew with a locking mechanism.

In a second embodiment, a method is provided. The method comprisesgenerating a laser beam; directing the laser beam at a nonlinear crystalconfigured for wavelength conversion; and nonlinearly converting thelaser beam. The nonlinear crystal is positioned such that a focus of thelaser beam is outside the nonlinear crystal in at least one planeperpendicular to a beam propagation direction of the laser beam. Thenonlinear conversion of the laser beam can be one of second harmonicgeneration, sum-frequency generation, or difference frequencygeneration.

DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating an embodiment of a wavelengthconverter in accordance with the present disclosure;

FIG. 2 is a schematic diagram illustrating a first embodiment of asystem in accordance with the present disclosure;

FIG. 3 is a schematic diagram illustrating a second embodiment of asystem in accordance with the present disclosure;

FIG. 4 is a schematic diagram illustrating a third embodiment of asystem in accordance with the present disclosure;

FIG. 5 is an exemplary focus size; and

FIG. 6 is a flowchart in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Although claimed subject matter will be described in terms of certainembodiments, other embodiments, including embodiments that do notprovide all of the benefits and features set forth herein, are alsowithin the scope of this disclosure. Various structural, logical,process step, and electronic changes may be made without departing fromthe scope of the disclosure. Accordingly, the scope of the disclosure isdefined only by reference to the appended claims.

The improved nonlinear optical wavelength converters disclosed hereinintroduce a degree of freedom to independently optimize the beam sizes,and, thus, the power densities both in the nonlinear crystal and on theadjacent optical elements of the beam shaping optics. In contrast toother wavelength converter devices, this optimization can be achievedwithout increasing the form factor of the device. Even for relativelylarge beam sizes inside the crystal, the beam divergence angle can bemade large to reduce the power density on the downstream optics. Inaddition, the beam size inside the nonlinear optical crystal can beadjusted as needed to scale the harmonic power with minimal changes tothe optics design.

The disclosure comprises a compact design for a nonlinear opticalwavelength converter that improves the optical performance as well asthe component lifetime. The disclosure allows an optics designer toindependently optimize the optical power density in or on the nonlinearcrystal and other components of the beam shaping optics. This isachieved by locating the nonlinear crystal outside the focus of thefundamental beam. Furthermore, the power density inside the nonlinearcrystal can be scaled via the beam size without redesigning thewavelength converter optics train by adjusting the out-of-focus positionof the nonlinear crystal. Due to this feature, an optimal power densityin the nonlinear crystal can be maintained when increasing the harmonicpower.

Nonlinear crystals can be used to create a UV laser beam by generating aharmonic of a long wavelength beam or by mixing two laser beams ofdifferent frequencies to create a frequency equal to the sum (ordifference) of the two frequencies. Thus, nonlinear wavelengthconversion, such as sum-frequency generation (SFG) and second harmonicgeneration (SHG) in nonlinear optical crystals, can generate laserradiation at wavelengths not directly accessible through the emissionlines of efficient solid state laser sources. This method can, forexample, be used to extend the wavelength range of diode-pumpedsolid-state lasers, emitting in the near infrared, into the visible, UV,and DUV. UV and DUV generation is typically achieved by cascading two ormultiple SHG and SFG steps. The third harmonic (THG) is, for example,generated by an SHG process followed by an SFG process, whereas thefourth harmonic (FHG) is generated in two cascaded SHG processes.Because the harmonic generation and the mixing process are non-linearprocesses, higher incident power density typically results in a moreefficient conversion process and higher output power.

In order to achieve efficient wavelength conversion in, for example, anSHG process, the dispersion between the fundamental and second harmonicis minimized. This can be achieved by choosing the propagation directionin a birefringent nonlinear crystal (i.e., the phase-matching angle) sothat the ordinary refractive index at the fundamental wavelength matchesthe extraordinary refractive index at the second harmonic wavelength orvice versa. For most phase matching angles, the pointing vector of thesecond harmonic beam inside the nonlinear crystal is not parallel to thewave vector. This condition is commonly referred to as critical phasematching. The pointing vector walk-off (“walk-off”) can be taken intoaccount for any nonlinear wavelength converter design, so that anydetrimental impact on the spatial beam quality can be minimized.

The small signal gain in SHG is proportional to the square of thefundamental power density. A Gaussian beam is focused into the nonlinearcrystal to maximize the power density throughout the crystal and achievemaximal conversion efficiency.

The optimal focus size depends on both, the length of the nonlinearcrystal and its walk-off angle. In case of critical phase matching thenonlinear conversion efficiency can be further increased by choosing anelliptical focus with a larger waist in the plane of the walk-off. Inaddition to achieving higher conversion efficiency, elliptical focusingcan be used to reduce the impact of the walk-off on the spatial beamquality. A software package to optimize the focusing conditions for bothcircular and elliptical focusing can be used.

In high-power applications, both the power and the spatial beam qualityof the second harmonic can be negatively impacted by detrimental effectsintroduced by the high power density (e.g., nonlinear absorption,thermal dephasing, and photorefraction). Furthermore, the high powerdensity in the nonlinear material can induce the formation of crystaldefects (such as excitons and color centers), photorefractive damage,and optically induced surface damage. These effects can result in adegradation of the nonlinear crystal quality over time. The effects areespecially prominent when the photon energy of the radiation is largeenough to allow for two-photon absorption in the material. Depending onthe nonlinear material that is used, this may be the case for radiationin the visible, UV, or DUV. Increasing the focus size inside the crystalcan reduce the power density. As this method reduces the conversionefficiency as well, a trade-off has to be made between nonlinearconversion efficiency, spatial beam quality, and longevity of thenonlinear crystal. In the case of critical phase matching, ellipticalfocusing generally yields an increased conversion efficiency andimproved beam quality at a given power density.

If the focus on the fundamental beam is located in the center of thenonlinear crystal, then the focus size of the fundamental beam must beincreased to increase the beam size in the nonlinear crystal. Thus, thefundamental focusing and harmonic beam shaping optics would need to beredesigned. If the form factor of the device is to remain unchanged,then the beam size on focusing and beam-shaping optics elementsdecreases. This increases the power density on the optics and,consequently, decreases lifetime of the optics.

Increasing the incident laser power on a nonlinear crystal can haveundesirable side effects. For example, permanent damage may occur in thecrystal over time. With accumulated exposure, this damage can result ingenerally decreasing power intensity as well as generally increasingastigmatism. Therefore, correcting the astigmatism with optics mayrequire frequent compensating adjustments, which would be impractical incommercial applications. Moreover, the astigmatism also may rapidlyincrease to the level where accurate compensation is not possible evenwith adjustment.

Generating a shorter output wavelength also can accelerate thedegradation of the crystal because the output photons are more energeticand, therefore, can change characteristics of or even permanently damagethe crystal. Thus, at shorter output wavelengths, astigmatism and otheradverse beam quality and intensity effects also may increasingly occur.

When scaling the harmonic power (e.g., by increasing the fundamentalpower as a higher-power fundamental laser source becomes available), thetrade-off between optical power, spatial beam quality, and crystallifetime may not be optimized, so that the beam size in the nonlinearcrystal has to be re-optimized. The focus size in the nonlinear crystalmay need to be changed to re-optimize. This can require a major opticaland optomechanical redesign of the nonlinear wavelength convertermodule.

FIG. 1 is a schematic diagram illustrating an embodiment of a wavelengthconverter 100. The wavelength converter 100 includes a nonlinear crystal101 and beam shaping optics 102. The beam shaping optics 102 can includeone or more lenses, mirrors, or other optical components. An additionalbeam shaping optics (not illustrated), which also can include one ormore lenses, mirrors, or other optical components, may be positioned onthe opposite side of the nonlinear crystal 101 from the beam shapingoptics 102.

A laser beam 103 is projected at the nonlinear crystal 101. The laserbeam 103 has a focus 104 that is outside the nonlinear crystal 101 in atleast one plane perpendicular to a beam propagation direction 109 of thelaser beam 103. For example, this plane may be parallel to the dashedlines representing the beam size 108. The distance between the center ofthe nonlinear crystal 101 and the focus 104 can be set or adjusted.

The focus 104 of the laser beam 103 is chosen to be small enough, and,thus, the beam divergence large enough, so that the fundamental and/orharmonic power density on one or multiple optical elements downstream orupstream of the focus 104 remains low enough to ensure a sufficientcomponent lifetime for the intended application (e.g., longer than oneyear or other periods of time). Depending on the wavelength offundamental and harmonic radiation and the material properties of thecomponent, the maximal allowable power density may be in a range from100 W/cm² to 1 MW/cm², including all values to the 1.0 W/cm² and rangestherebetween.

For any given Gaussian or near-Gaussian beam, the beam diameters,measured along two mutually perpendicular axes x and y, both also beingperpendicular to the beam propagation direction 109, are functions ofthe distance (z−z0) to the focus locations, as seen in Equation 1 andEquation 2

$\begin{matrix}{{w_{x}(z)} = {w\; 0_{x}*\sqrt{1 + \left( \frac{M^{2}{\lambda\left( {z - {z\; 0_{x}}} \right)}}{\pi\; w\; 0_{x}^{2}} \right)^{2}}}} & {{Eq}.\mspace{14mu} 1} \\{{w_{y}(z)} = {w\; 0_{y}*\sqrt{1 + \left( \frac{M^{2}{\lambda\left( {z - {z\; 0_{y}}} \right)}}{\pi\; w\; 0_{y}^{2}} \right)^{2}}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$with 2*w(z) being the beam diameter at the location z, defined as 4× thestandard deviation of the power distribution across the beam profile,2*w0 being the beam waist diameter, z0 being the beam waist location inthe direction of beam propagation, λ being the wavelength of theradiation, and M² being the beam propagation parameter. Thetime-averaged fundamental power density D_(F) at the spatial peak of thebeam profile can be calculated as a function of the beam radii, as seenin Equation 3

$\begin{matrix}{{D_{F}(z)} = \frac{2\; P_{F}}{\pi\; w\; 0_{x}w\; 0_{y}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$with P_(F) being the time-averaged fundamental power. If, for example,the nonlinear optical process is second harmonic generation, the beamsizes and the power density of the harmonic beam and the conversionefficiency for a given crystal position can be calculated based on thefundamental beam sizes described in Equations 1 and 2. In the presenceof walk-off this can be achieved by using a numeric simulation of theharmonic generation for focused Gaussian beams. Similar numerical modelsare available for other nonlinear optical wavelength conversionprocesses. The crystal position z and thus the beam size can be chosenso that the fundamental power density remains high enough to achieve thedesired conversion efficiency and harmonic output power, while, withinthe limits of this boundary condition, both fundamental and harmonicpower densities are minimized, so that the spot lifetime of thenonlinear crystal 101 is maximized. Typical distances between a positionof the focus 104 and a location of the nonlinear crystal 101 range frommillimeters to tens of centimeters. However, other distances arepossible.

In an example, the power of the harmonic radiation with a wavelength of266 nm, generated in a nonlinear crystal 101, such as BBO, is doubledwhile fundamental and harmonic power density in the nonlinear crystal101 remain unchanged. Under these conditions the crystal spot lifetimeremains unchanged as well. This can be achieved by doubling power of thefundamental laser(s), as well as the beam area inside the nonlinearcrystal 101. For example, an elliptical fundamental beam size of 450μm×200 μm may be increased to 450 μm×400 μm. In this example the majoraxis of the ellipse is parallel to the walk-off. The beam size in thenonlinear crystal 101 can be increased by increasing the size of a focuslocated inside the nonlinear crystal 101. As a result the beam area on adownstream optics at a distance of, for example, 0.5 m decreases by afactor of 1.8×, so that the harmonic power density on or in the opticsincreases by a factor of 3.6×. If the downstream optics experiences adamage mechanism, which scales with the square of the DUV power density,the optics lifetime decreases by a factor of more than 10×.

When using the embodiments disclosed herein, the same DUV power and DUVpower density in the nonlinear crystal 101 can be achieved bymaintaining the original focus size and position and moving thenonlinear crystal 101 downstream of the focus 104 by 0.1 m. As there isno change to the focus size, the fundamental and harmonic beam sizes onthe downstream mirror remain the same (in the absence of secondaryeffects such as thermal lensing), so that the DUV power density on themirror increases only by a factor of 2×, resulting in less of an impactto optics lifetime. This is a significant improvement over previoussystems. The distance the nonlinear crystal 101 is moved upstream ordownstream of the focus 104 can vary, and moving the nonlinear crystal101 downstream of the focus 104 by 0.1 m is merely one example.

The nonlinear crystal 101 can be configured to provide phase matching toachieve efficient nonlinear interactions in a medium. The nonlinearcrystal 101 may utilize critical phase matching, noncritical phasematching, quasi-noncritical phase matching, or quasi-phase matching.

The nonlinear crystal 101 may be or include BBO or CsLiB₆O₁₀ (CLBO) forDUV applications. However, other types of nonlinear crystals may beused, such as those that are or include LiIO₃, KNbO₃, monopotassiumphosphate (KH₂PO₄), lithium triborate (LBO), GaSe, potassium titanylphosphate (KTP), lithium niobate (LiNbO₃), LiIO₃, or ammonium dihydrogenphosphate (ADP).

Nonlinear crystals, such as the nonlinear crystal 101, are typicallygrown in boules, and then cut into individual crystal elements. Theinput and output surface are polished after cutting. The dimensions ofthe available crystal elements depend on the properties, such as boulesize and boule quality, of the chosen nonlinear optical material.Nonlinear optical crystals may have length dimensions from 1 mm to 50mm, width dimensions from 3 mm to 20 mm, and height dimensions from 0.5mm to 10 mm, including all values to the 0.01 mm and rangestherebetween. The direction of beam propagation, such as beampropagation direction 109, is typically referred to as a “length.” Othernonlinear crystal dimensions are possible for different applications. Anonlinear crystal 101 of any size suitable for a desired application canbe used in the embodiments disclosed herein.

As seen in FIG. 1, the beam size 107 (represented with dashed lines) ator in the nonlinear crystal 101 is greater than the beam size 108(represented with dashed lines) at the focus 104 because the nonlinearcrystal 101 is positioned downstream of the focus 104. The power densityat the beam size 107 is less than the density at the beam size 108.Thus, the nonlinear crystal 101 is affected by part of the laser beam103 with a lower power density.

The beam size 107 inside the nonlinear crystal 101 can be optimized byadjusting the position of the nonlinear crystal 101 outside the focus104. When using techniques disclosed herein, changes to the size andlocation of the focus 104 of the laser beam 103 can be avoided. As thewaist size and location of the focus 104 can remain unchanged, the beamsize on the beam shaping optics 102 downstream of the nonlinear crystal101 may remain unchanged. More particularly, the beam size on the beamshaping optics 102 downstream of the nonlinear crystal 101 may notdecrease.

Downstream of the nonlinear crystal 101, the laser beam 103 includes afundamental beam 105 and a harmonic beam 106. While the harmonic beam106 is illustrated in a particular manner in FIG. 1, the beam sizes ofthe harmonic beam 106 may be smaller than the fundamental beam sizes 105by a factor of 1.41× (i.e., √2).

The wavelength converter 100 can use a divergent or convergent beaminside the nonlinear crystal 101. In one plane (parallel orperpendicular to the walk-off) or in both planes, the fundamental beam105 is focused outside the crystal as shown in FIG. 1. The size of thefocus 104 can be chosen so that it provides a short enough Rayleighrange, and, thus, a large enough beam divergence to reduce the powerdensity on the beam shaping optics 102 to a level that provides thedesired optics lifetime. In this case, the Rayleigh range is thedistance along the beam propagation direction 109 from the waist to theplace where the beam width is increased by a factor of √2. If powerscalability is intended for the wavelength converter 100, the beam sizeon the beam shaping optics 102 is chosen to be large enough so that therequired maximum power density on the beam shaping optics 102 is notexceeded for the highest intended fundamental and harmonic powers thatwill be present in a power-scaled version of the wavelength converter.To avoid any power and/or beam quality penalty in the case of criticalphase matching, the beam divergence (or convergence) within thenonlinear crystal 101 in the plane parallel to the walk-off, may remainbelow the limit set by the crystal angular acceptance. The margindepends on the specific beam quality requirement for an application.

The divergence or convergence of a laser beam can be described byGaussian beam propagation. The beam divergence parallel to a givenlateral axis at a given position z along the beam propagation directionis the arctangent of the first derivative of Equations 1 and 2.Therefore, the divergence angle at a given location can be decreased byincreasing the waist size. The acceptance angle of the nonlinear processis defined as the angle offset from the optimum phase-matching angle inthe critical direction of phase matching, for which the nonlinearconversion efficiency is reduced to 50% of the conversion efficiency atthe optimal phase matching condition. Typical values of the angularacceptance, which is defined as a half angle herein, for harmonicgeneration into the DUV are on the order of 0.05 to 0.5 mrad*cm ofcrystal length. The acceptance angle can depend on the nonlinearprocess, the nonlinear crystal material being used, and the length ofthe nonlinear crystal. The acceptance angle can be calculated based onthe Sellmeier equations for the nonlinear crystal material.Alternatively a software package can be used to calculate the angularacceptance. In order to reduce the conversion efficiency loss due to thebeam divergence angle to less than 4%, the beam divergence angle,defined as 2× the standard deviation to the angular laser powerdistribution, in the critical direction of phase matching may be limitedto less than 50% of the acceptance angle as defined above.

The nonlinear crystal 101 position along the axis of the beampropagation direction 109 can be chosen so that the power density in thenonlinear crystal 101 meets the requirements needed to achieve thedesired trade-off between one or more of or between two or more ofconversion efficiency, spatial beam quality, crystal lifetime, orcrystal spot lifetime.

For nonlinear wavelength conversion, the conversion efficiency increaseswith increasing power density, so that an as small as possible beam sizein the nonlinear crystal 101 may be desirable to maximize the conversionefficiency. On the other hand, the nonlinear crystal 101 may experiencedamage induced by the generated harmonic or even by the fundamentalradiation, so that crystal spot used for wavelength conversion has alimited lifetime. The exact scaling laws can depend on the specificdamage mechanism experienced by the crystal. However, the crystallifetime decreases with increasing power density, so that an as large aspossible beam size in the crystal may be desirable to maximize the spotlifetime.

A nonlinear crystal spot shift may trigger a service-event for thewavelength converter, so maintaining a large enough spot lifetime (e.g.,in the range of several hundreds of hours or longer) can achieve thedesired service interval. The crystal lifetime, as a whole, is the sumof the spot lifetimes for all crystal spots. If the spot lifetimedecreases linearly with decreasing spot size (i.e., increasing powerdensity), the crystal lifetime becomes independent of the spot size, asthe number of available spots increases at the same rate as theindividual spot lifetime decreases. For damage mechanisms that follow ascaling law, which is faster than linear (e.g., a spot lifetime thatdecreases inversely proportional to the square of the power density),increasing the beam size improves the crystal lifetime because thenumber of available spots decreases linearly with the increasing beamarea, whereas the lifetime of the individual crystal spots increasesfaster than linearly with increasing spot size (e.g., proportional tothe square of the beam area in the above example).

The nonlinear crystal 101 provides wavelength conversion of the laserbeam 103. The optimal beam size in the nonlinear crystal 101 can bechosen to maximize the achievable conversion efficiency, and, thus,maximize the power at the harmonic wavelength, while maintaining arequired spot lifetime and/or crystal lifetime. Alternatively, theoptimal beam size in the nonlinear crystal 101 may be chosen to maximizethe achievable spot lifetime and/or crystal lifetime while achieving theconversion efficiency and, thus, the power at the harmonic wavelengththat is required for an application.

The wavelength converter 100 enables scaling of the second harmonicpower by increasing the fundamental power from the primary laser sourceused to generate the laser beam 103. The optimum power density can bemaintained by moving the nonlinear crystal 101 farther away from thefocus 104. Changes to the fundamental focusing optics design may not beneeded. Minor changes in the harmonic beam shaping optics may still beperformed to compensate for possible effects induced by the differentpositions of the nonlinear crystal 101 and beam size in the nonlinearcrystal 101. For example, the changed location and power of a possiblethermal lens inside the crystal may be compensated for. However, thesechanges are minor compared to a complete redesign of the wavelengthconverter optics train. Such changes can be accommodated by takingadvantage of adjustable beam shaping components for the harmonic beam,such as an adjustable beam expanding telescope or a Cooke triplet, inthe downstream beam shaping optics 102.

In an instance, a focus 104 for a particular nonlinear crystal 101 thatplaces the nonlinear crystal 101 outside the focus 104 and provide alaser beam 103 with desired parameters can be determined in a two-stepprocess. First, the fundamental beam size in the nonlinear crystal 101,as a function of the nonlinear crystal 101 location, can be determinedby using the techniques disclosed herein. Second, the harmonic beam sizecan be calculated based on the fundamental beam size. Using thesecalculations the distance between the focus 104 and the nonlinearcrystal 101 can be chosen so that the fundamental and harmonic powerdensities meet specifications for a particular application.

FIG. 2 is a schematic diagram illustrating an embodiment of a system200. A laser source 201 containing a laser active medium generatesfundamental radiation, such as the laser beam 103, in the beampropagation direction 206. The laser source 201 may be, for example, asolid state laser, semiconductor laser, gas laser, fiber laser, CWlaser, mode-locked laser, Q-switched laser, gain-switched laser, laserwith a built-in nonlinear wavelength converter, or another type oflaser. The laser beam 103 emitted by the laser source 201 may be adiffraction-limited or near diffraction-limited Gaussian beam. Othertypes of laser beams 103 are possible.

In an instance, the laser source 201 is an exchangeable laser source. Anexchangeable laser source that is part of the nonlinear optical systemcan be exchanged with a laser source of identical design as a fieldreplaceable unit upon its failure or once it reaches the end of itsservice lifetime. An exchangeable laser source that is part of thenonlinear optical system also can be exchanged with a laser source of adifferent design, such as a higher power laser source, to improve theperformance (e.g., the output power of the nonlinear optical system). Inthis case, the out-of-focus position of the nonlinear crystal 101 can beadjusted, as described herein, to achieve an optimal trade-off betweenharmonic power and lifetime of the nonlinear crystal 101.

The laser beam 103 projects through beam shaping optics 202 upstream ofthe nonlinear crystal 101. The beam shaping optics 202 can include oneor more lenses, mirrors, or other optical components. The beam shapingoptics 202 may comprise a single lens or multiple lenses and maygenerate a circular or elliptical focus with or without astigmatism. Thebeam shaping optics 202 may or may not be adjustable. The beam shapingoptics 202 may be located between the laser source 201 and the nonlinearcrystal 101 or may be integrated into the laser source 201. A focus 104of the laser beam 103 is outside the nonlinear crystal 101 in at leastone plane perpendicular to a beam propagation direction 206 of the laserbeam 103. The location and size of the focus 104 can vary. Typical focusdiameters may range from 5 μm to approximately 1 mm, including allvalues to the 1 μm and ranges therebetween. Typical distances betweenthe focus 104 position and the nonlinear crystal 101 range frommillimeters to tens of centimeters. However, focus 104 sizes anddistances to the nonlinear crystal 101 outside of this range arepossible.

The nonlinear crystal 101 is disposed in a crystal mount assembly 203.Thus, the nonlinear crystal 101 may be on or in the crystal mountassembly 203. The crystal mount assembly 203 may be fabricated of metalsuch as, but not limited to, aluminum, stainless steel, copper,copper-tungsten, or nickel. The crystal mount assembly 203 also may befabricated of ceramics or other materials.

The crystal mount assembly 203 can be designed to keep the position ofthe nonlinear crystal 101 stable within, for example, tens of micronsand the angle relative to the incident laser beam stable within, forexample, 27% of the angular acceptance range during shipment andoperation of the nonlinear wavelength converter. An angle change of 27%of the angular acceptance in the walk-off direction can result in a 5%drop of the conversion efficiency relative to the optimal phase matchingangle.

The crystal mount assembly 203 may be a spring-loaded assembly, whereinthe nonlinear crystal 101 is positioned in an L-bracket and held inplace by springs along one or multiple axes perpendicular to the beampropagation direction 109. The springs press the nonlinear crystal 101onto or against the L-bracket.

Due to the limited angular acceptance of the nonlinear opticalinteraction, the angle tolerance may be in the range from 0.03 mrad to0.3 mrad. Therefore, the crystal mount assembly 203 may contain featuresto adjust the phase matching angle of the nonlinear interaction. Suchfeatures include, but are not limited to rotation stages actuated by amanual fine thread screws, manual micrometer actuators, manualdifferential micrometer actuators, or motorized actuators. The entirecrystal mount assembly 203 may be rotated during the adjustment of thecrystal phase matching angle. The crystal mount assembly 203 may containa locking mechanism to lock the rotation angle of the nonlinear crystal101 once the alignment is completed. Such locking mechanisms include,but are not limited to, screws that are tightened in a directionperpendicular to the direction of rotation to press the rotating part ofthe stage against a surface providing sufficiently high friction. Fornonlinear crystals 101 with typical dimensions, the size of the crystalmount assembly 203 is typically in the range from 10 mm to 150 mm ineither direction. Other dimensions of the crystal mount assembly 203 arepossible and these ranges are merely exemplary.

In addition the crystal mounting assembly may contain manual ormotorized translation stages so that the nonlinear crystal 101 can bemoved perpendicular and/or parallel to the beam propagation direction109.

The crystal mount assembly 203 can be temperature controlled. Inaddition to angular alignment, the phase matching condition is sensitiveto the temperature of the nonlinear crystal 101 because the refractiveindices at the fundamental and the harmonic wavelength have differenttemperature dependences. The temperature acceptance can be defined asthe temperature range around the optimal phase matching temperature inwhich the nonlinear optical conversion efficiency is higher than 50% ofthe conversions efficiency at the optimal phase matching temperature.Typical temperature acceptance ranges for nonlinear crystals used togenerate DUV radiation are on the order of 6° C.*cm of crystal length.To maintain a conversion efficiency larger than 95% of the value atoptimal phase matching temperature, it may be necessary to maintain thecrystal temperature in a range less than 25% of the temperatureacceptance (i.e., within a range of 1.5° C.*cm of crystal length). Tokeep the crystal temperature in the described optimal range, the crystaltemperature may be actively controlled. This can be achieved by using atemperature controller including a sensor, such as, but not limited to,a thermocouple, a thermistor, or an RTD temperature sensor, to measurethe temperature. The temperature controller also can include a heater ora Peltier element electronically connected to the sensor to adjust thetemperature. Both the sensor and temperature controller can beintegrated in or attached to the crystal mount assembly 203. Thetemperature controller also may include aproportional-integral-derivative (PID) controller to establish atemperature control loop.

The crystal mount assembly 203 can contain a crystal enclosure and/or acrystal oven. Several nonlinear crystal materials, including LBO, CLBO,and LiIO₃, are highly hygroscopic. It can be advantageous to protectsuch nonlinear crystals from environmental humidity using an enclosure.The enclosure may be hermetically sealed or actively purged with a drypurge gas, such as nitrogen, argon, or clean, dry air. The enclosurebody may be made of metal or ceramics. The enclosure has two windows toallow for the entry of the fundamental laser beam and an exit of boththe fundamental and harmonic beam. The windows may comprise a substratematerial transmitting both the fundamental and harmonic wavelength.Possible window materials include, but are not limited to, fused silica,calcium fluoride, magnesium fluoride, or crystalline quartz. Theinterface between the body of the enclosure and the windows may besealed using a seal, such as an O-ring seal or other types of seals. Thecrystal enclosure may include a crystal oven or a temperature controlledcrystal mount.

To minimize humidity exposure of hygroscopic crystals, the crystal maybe operated at an elevated temperature to minimize the risk ofcondensation of environmental humidity on the crystal surfaces. Theelevated temperature may be, for example, from 40° C. and 200° C. Thecrystal may be heated using a crystal oven. The oven body may befabricated of metal (e.g., aluminum or copper) or of ceramics. Thedesign may be similar to the temperature controlled crystal mountdescribed herein. Features may be added to optimize the oven for hightemperature operation, such as resistive heaters that are optimized forhigh-temperature operation, or thermal insulating features to improvethe homogeneity of the temperature distribution inside the oven. Suchinsulating features may include thermal insulation layers (e.g., thoseincluding ceramics or fluoropolymers).

The crystal mount assembly 203 can be configured to allow exchange ofthe nonlinear crystal 101 as it reaches the end of its lifetime.

The crystal mount assembly 203 is at a location along the beampropagation axis 206 so that the focus 104 in at least one of the planesperpendicular to the beam propagation axis 206 is outside of thenonlinear crystal 101. The crystal mount assembly 203 can be fixed orcan be translatable perpendicular and/or parallel to the beampropagation direction 206. In the embodiment of FIG. 2, the crystalmount assembly 203 is attached or fixed to a wall of the system 200 at adesired location.

The laser beam 103 passes through the beam shaping optics 102 downstreamof the nonlinear crystal 101. The beam shaping optics 102 can includeone or more lenses, mirrors, or other optical components. The beamshaping optics 102 may or may not be adjustable.

In an instance, the beam shaping optics 102 are or include beam shapingoptics for the harmonic beam generated in the nonlinear crystal 101.This harmonic beam shaping optics may include adjustable opticalelements and/or adjustable optomechanical elements.

In an example, the laser beam 103 is used to image a wafer 204 disposedon a stage 205. However, the laser beam 103 can be used in otherapplications or with other workpieces.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium nitride, gallium arsenide, indiumphosphide, sapphire, and glass. Such substrates may be commonly foundand/or processed in semiconductor fabrication facilities.

A wafer may include one or more layers formed upon a substrate. Forexample, such layers may include, but are not limited to, a photoresist,a dielectric material, a conductive material, and a semiconductivematerial. Many different types of such layers are known in the art, andthe term wafer as used herein is intended to encompass a wafer includingall types of such layers.

One or more layers formed on a wafer may be patterned or unpatterned.For example, a wafer may include a plurality of dies, each havingrepeatable patterned features or periodic structures. Formation andprocessing of such layers of material may ultimately result in completeddevices. Many different types of devices may be formed on a wafer, andthe term wafer as used herein is intended to encompass a wafer on whichany type of device known in the art is being fabricated.

As seen in FIGS. 3 and 4, the crystal mount assembly 203 may beconfigured so that the crystal mount assembly 203 can be positioned inone or multiple locations outside the fundamental focus, so thatfundamental beam size can be adjusted to achieve an optimal trade-offbetween conversion efficiency, spatial beam quality, crystal lifetime,or crystal spot lifetime without having to change the alignment of thebeam shaping optics 102, 202. Thus, the crystal mount assembly 203 canbe configured to adjust a distance between a center of the nonlinearcrystal 101 and the focus 104.

FIG. 3 is a schematic diagram illustrating an embodiment of a system300. The system includes one or more mounting features 301. While fourmounting features 301 are illustrated in FIG. 3, more or fewer mountingfeatures 301 are possible. These mounting features 301 may be arrangedin an array. The mounting features 301 are arranged at differentdistances from the laser source 201 or between the beam shaping optics102, 202. The crystal mount assembly 203 is configured to be disposed onor in one of the mounting features 301. A beam size of the laser beam103 in the nonlinear crystal 101 is provided by selecting one of themounting features 301 closer to or farther from the focus 104. Thus, themounting features 301 are configured to adjust a beam size of the laserbeam 103 in the nonlinear crystal 101 by allowing adjustment of adistance between a center of the nonlinear crystal 101 and the focus104.

In an example, each of the mounting features 301 includes one or moreslots or holes configured to receive a component of the crystal mountassembly 203. Each of the mounting features 301 can be positioned suchthat the focus 104 is outside the nonlinear crystal 101 in at least oneplane perpendicular to a beam propagation direction 109 of the laserbeam 103. For example, the mounting feature 301 can be a fastener holethat the crystal mount assembly 203 is fastened into. The crystal mountassembly 203 can be screwed into the mounting feature in an instance.

Different beam sizes in the nonlinear crystal 101 can be generated bymoving the crystal mount assembly 203 to different locations downstreamor upstream of the smaller focus in the non-walk-off direction. With thesystem 300, the ability to adjust the beam size in the nonlinear crystal101 simplifies power scaling the DUV light source, such as when higherpower laser sources 201 and/or nonlinear crystals 101 with improvedmaterial quality are available. The focus 104 can be outside thenonlinear crystal 101 with a Rayleigh range configured to limitfundamental optical power density or harmonic optical power density onor in at least one optical component in the system and/or to optimizethe beam size inside the nonlinear crystal 101.

FIG. 4 is a schematic diagram illustrating an embodiment of a system400. The crystal mount assembly 203 is configured to adjust a beam sizeof the laser beam 103 in the nonlinear crystal 101 by adjusting adistance between a center of the nonlinear crystal 101 and the focus 104using adjustment assembly 401. The adjustment assembly 401 is connectedto the crystal mount assembly 203. The adjustment assembly 401 canprovide additional degrees of freedom, accuracy, and/or precisionrelative to the configurations of FIG. 2 or FIG. 3. The adjustmentassembly 401 can move the crystal mount assembly 203 in one, two, orthree axes.

The adjustment assembly 401 can be, for example, a fine-thread screwwith a locking mechanism, a micrometer screw with a locking mechanism,an actuator, or a robotic system. The fine-thread screw or micrometerscrew may be hand-actuated or automated.

The embodiments disclosed herein can contain power sensors andcomponents to implement a control loop for the harmonic power. This maybe, for example, a light loop. Such a control loop may use one or morecomponents, such as polarizers, waveplates, acousto-optic modulators,electro-optic modulators in the path of the fundamental or the harmonicbeam, or an electronic feedback to the laser source 201 to control thefundamental or the harmonic power.

The control loop can keep the harmonic output power constant. To achievethis, the harmonic power can be measured near the beam output using asensor, such as a photodiode, a thermopile sensor, or other types ofsensors. The control loop compares the measured harmonic output powerwith the power target and adjusts the harmonic output power accordingly.This adjustment can be achieved in different manners.

The harmonic power can be adjusted directly by using a modulator in theharmonic beam path downstream of the nonlinear crystal 101. Suitablemodulators include, but are not limited to, acousto-optic modulators,electro-optic modulators, as well as variable attenuators comprising acombination of a rotatable half-wave-plate and a polarizer.

Alternatively the modulators can be located in the fundamental beam pathupstream of the nonlinear crystal 101.

The temperature of the nonlinear crystal 101 may be adjusted to tune ordetune the phase matching, and, thus, adjust the nonlinear conversionefficiency and the output power.

The control signal may be communicated back to the laser source, so thatthe laser output power can be adjusted in order to adjust the generatedharmonic power.

In any embodiment disclosed herein, the nonlinear crystal 101 can berepositioned or adjusted within the crystal mount assembly 203 and/orthe crystal mount assembly 203 can be repositioned relative to the laserbeam 103 so that a non-damaged part of the nonlinear crystal 101receives the laser beam 103 if the nonlinear crystal 101 is damaged. Forexample, the crystal mount assembly 203 and/or the nonlinear crystal 101can be moved in one or two directions perpendicular to the beampropagation direction 206. The focus 104 can be outside the nonlinearcrystal 101 with a Rayleigh range configured to limit fundamentaloptical power density or harmonic optical power density on or in atleast one optical component in the system and/or to optimize the beamsize inside the nonlinear crystal 101.

The nonlinear crystal 101 can be positioned in a laser beam 103 that isdivergent in the axis perpendicular to the walk-off or in both axes. Fora given beam size in the center of the nonlinear crystal 101, thisreduces the power density on the crystal output facet and may increasethe crystal spot lifetime.

A focus in the embodiments disclosed herein may be circular orelliptical. An elliptical focus may have a longer Rayleigh range in onedirection than in another direction, which makes the beam size change afunction of the nonlinear crystal 101 distance from the focus 104 lesssensitive in the direction of the longer Rayleigh range than in thedirection of the shorter Rayleigh range. With an elliptical focus, thesmaller waist of the elliptical focus can be located outside thenonlinear crystal 101. For a nonlinear crystal 101 with critical phasematching, an elliptical focus with a larger waist diameter in the planeparallel to the walk-off direction can be used so that the focus in theplane parallel to the walk-off can be larger than the focus in the planeperpendicular to the walk-off.

The focus also may be astigmatic. With an astigmatic focus, the focus inone plane can be inside the nonlinear crystal 101 and the focus inanother plane is outside the nonlinear crystal 101.

In an example, the focus is astigmatic and elliptical. In this example,the focus in one plane is inside the nonlinear crystal 101 and the focusin another plane is outside the nonlinear crystal 101. The focus insidethe nonlinear crystal 101 has a larger width than the focus outside thenonlinear crystal 101.

In an example, the laser beam 103 is diffraction limited ornear-diffraction-limited and a nonlinear crystal 101 with critical phasematching is used. In this example, a focus size in the directionparallel to the walk-off is large enough at all of the desiredout-of-focus locations of the nonlinear crystal 101 so that the impactof the walk-off on the beam quality can be minimized. For tightlyfocused beams, which have a large divergence, the walk-off can createside lobes of the beam in the walk-off direction that cause deviationsfrom an ideal Gaussian beam shape, and, thus, have a negative impact onthe spatial beam quality in the walk-off direction. This can bedetrimental for applications (e.g., in wafer inspecting) that rely on anideal Gaussian beam shape. The far-field beam shape of the generatedharmonic beam for a given crystal length, beam size, and walk-off anglecan be simulated. Based on the simulation, the deviation of the farfield profile from an ideal Gaussian profile can then be determined forany lateral position across the beam profile. Based on this calculationa waist size can be chosen, so that the maximum deviation of the lateralfar field profile from a Gaussian profile is smaller than a specifiedvalue. This specified value may be, for example, 4%.

In another example, the laser beam 103 is diffraction limited or neardiffraction-limited and a nonlinear crystal 101 with critical phasematching is used. The nonlinear crystal 101 location is adjustable. ARayleigh range in the direction parallel to the walk-off is large enoughso that the laser beam 103 divergence in the direction parallel to thewalk-off remains small enough for all of the desired out-of-focuslocations of the nonlinear crystal 101 so that the impact of the crystalangular acceptance on the spatial beam quality can be minimized.

FIG. 5 is an exemplary focus 104 size. The size of the focus 104 of thelaser beam 103 in the plane parallel to the walk-off (dashed lines) islarger than that in the plane perpendicular to the walk-off (solidlines). The foci in both planes are located outside of the nonlinearcrystal 101. The focus in the walk-off direction is larger than thefocus in the non-walk-off direction. The beam propagation in thewalk-off direction is described by the lines with the smaller curvature,whereas the propagation in the non-walk-off direction shows a smallerand more pronounced beam waist.

In an example, the nonlinear crystal is positioned such that the focusof the laser beam is outside the nonlinear crystal in the at least oneplane perpendicular to the beam propagation direction of the laser beamwith a Rayleigh range configured such that time averaged fundamentaloptical power density or harmonic optical power density at the spatialpeak of the beam profile on or in at least one optical component in thesystem is limited to below 1 MW/cm². The crystal mount assembly can beadjustable or can include a plurality of mounting features at differentdistances from the laser source in this example.

In another example, the laser beam is a pulsed laser beam. The nonlinearcrystal is positioned such that the focus of the pulsed laser beam isoutside the nonlinear crystal in the at least one plane perpendicular tothe beam propagation direction of the pulsed laser beam with a Rayleighrange configured such that the fundamental optical fluence or harmonicoptical fluence on or in at least one optical component of the system islimited to below 10 J/cm². Other values of fundamental optical fluenceor harmonic optical fluence are possible. For example, the fundamentaloptical fluence or harmonic optical fluence on or in at least oneoptical component may be from 1 J/cm² to 20 J/cm², including all valuesto the 0.5 J/cm² and ranges therebetween. The crystal mount assembly canbe adjustable or can include a plurality of mounting features atdifferent distances from the laser source in this example.

FIG. 6 is a flowchart of a method 500. The method 500 includesgenerating a laser beam 501. The laser beam is directed at a nonlinearcrystal 502. This may be through, for example, beam shaping optics. Thelaser beam passes through the nonlinear crystal 503. A focus of thelaser beam is outside the nonlinear crystal in at least one planeperpendicular to a beam propagation direction of the laser beam. Thelaser beam is nonlinearly converted 504. The nonlinear conversion of thelaser beam can be SHG, SFG, or difference frequency generation (DFG).The crystal mount assembly can be adjustable or can includes a pluralityof mounting features at different distances from the laser source.

The focus 104 also can be located downstream of the nonlinear crystal101 and provide many of the advantages disclosed herein. The beam sizeon the crystal output facet is smaller with a downstream focus than inthe case of an upstream focus. Thus, the power density on the outputfacet is higher and the crystal lifetime may be shorter. This effect isgenerally minimal, so either an upstream focus or a downstream focus maybe used with the embodiments disclosed herein.

The nonlinear wavelength conversion process and the embodimentsdescribed herein can be SHG, SFG, or difference frequency generation.Thus, the wavelength conversion process can include one or more inputlaser beams.

The embodiments disclosed herein provide multiple advantages orbenefits. First, the beam size can be independently optimized. Thus, theoptical power density in the nonlinear crystal and on components of thebeam shaping optics can be optimized. A small focus (in at least oneplane) increases the divergence and convergence of the harmonic laserbeam inside a wavelength converter, such as the wavelength converter100. Therefore the power density on the beam shaping optics can bedecreased without increasing the dimensions of the wavelength converter.This increases the optics lifetime. A laser beam, described by Gaussianbeam propagation, has a constant product of waist diameter d0 andfar-filed divergence angle θ_(FF), so that d0*θ_(FF)=M²*λ/(2*π), withM²≥1 being the beam propagation parameter and λ being the wavelength ofthe radiation. For a given laser beam, the product of d0 and θ_(FF)parallel to each axis and perpendicular to be beam propagation directionis a constant, so the far field divergence angle increases as the waistdiameter decreases. Therefore, the divergence of the fundamental beamincreases when the waist size is decreased.

As the divergence of the fundamental beam increases, the divergence ofthe harmonic beam increases as well. For the example of SHG, thedivergence of fundamental and harmonic beam are identical in thenon-walk-off direction. In the walk-off direction divergence offundamental and harmonic beams are not identical due to the presence ofthe walk-off. However, an increased divergence of the fundamental beamstill results in an increased divergence of the harmonic beam.

Second, by positioning the nonlinear crystal outside the focus, thepower density inside the nonlinear crystal decreases. This increases thenonlinear crystal lifetime and improves the spatial beam quality of theharmonic radiation. Both the power density inside the nonlinear crystaland on the beam shaping optics can be optimized independently of oneanother without increasing the form factor of the wavelength converter.

Third, the power density in the nonlinear crystal can be adjustedwithout optics redesign by moving the nonlinear crystal along the beampropagation direction. This simplifies two power scaling options. In aninstance, the harmonic power can be increased by increasing thefundamental power while maintaining the same power density. For example,fundamental power can be increased by using a more powerful laser. Thisis achieved by moving the nonlinear crystal away from the focus, and,thus, increasing the beam size inside the nonlinear crystal. In anotherinstance, the harmonic power can be increased by increasing the powerdensity inside the crystal and, thus, the conversion efficiency when anonlinear crystal with a higher damage threshold becomes available. Thisis achieved by moving the nonlinear crystal toward the focus and, thus,reducing the beam size inside the nonlinear crystal.

Fourth, this can be used to retrofit existing tools because minimalchanges to the beam shaping optics are necessary. The wavelengthconverter, such as the wavelength converter 100, being part of a system,can be modified or reconfigured as part of a field-upgrade without anychanges or with minimal changes to the optics design.

Fifth, the position of the nonlinear crystal can be moved depending onthe material in the nonlinear crystal or the power density of the laserbeam. The position of the nonlinear crystal can be adjusted or optimizedif, for example, the type of or properties of laser beam is changed.

Sixth, the embodiments disclosed herein may provide improved performanceor lifetime with pulsed laser beams. A pulsed laser can causesignificant damage to a nonlinear crystal at high power densities.Changing the power density in the nonlinear crystal can enable use ofpulsed laser beams with less damage to the nonlinear crystal.

Although the present disclosure has been described with respect to oneor more particular embodiments, it will be understood that otherembodiments of the present disclosure may be made without departing fromthe scope of the present disclosure. Hence, the present disclosure isdeemed limited only by the appended claims and the reasonableinterpretation thereof.

What is claimed is:
 1. A method comprising: generating a laser beam;directing the laser beam at a nonlinear crystal configured forwavelength conversion, wherein the nonlinear crystal is positioned suchthat a focus of the laser beam is outside the nonlinear crystal in atleast one plane perpendicular to a beam propagation direction of thelaser beam, wherein the nonlinear crystal is disposed on a crystal mountassembly, wherein the crystal mount assembly provides an angle stabilityof greater than 0% and less than or equal to 27% of an angularacceptance range during operation, wherein the crystal mount assemblyincludes a plurality of mounting features at different distances from alaser source in the beam propagation direction, wherein each of themounting features includes a hole that the crystal mount assembly isfastened into, and wherein a beam size of the laser beam in thenonlinear crystal is provided by selecting one of the mounting features;and nonlinearly converting the laser beam from a first wavelength to asecond wavelength with the nonlinear crystal.
 2. The method of claim 1,wherein the nonlinearly converting is one of second harmonic generation,sum-frequency generation, or difference frequency generation.
 3. Themethod of claim 1, further comprising directing the laser beam throughbeam shaping optics disposed between a laser source and the nonlinearcrystal and disposed downstream of the nonlinear crystal in the beampropagation direction.
 4. The method of claim 1, further comprisingadjusting a beam size of the laser beam in the nonlinear crystal withthe crystal mount assembly by adjusting a distance between a center ofthe nonlinear crystal and the focus.
 5. The method of claim 1, whereinthe nonlinear crystal is positioned such that the focus of the laserbeam is outside the nonlinear crystal in the at least one planeperpendicular to the beam propagation direction of the laser beam with aRayleigh range configured such that time averaged fundamental opticalpower density or harmonic optical power density at the spatial peak ofthe beam profile on or in at least one optical component in the systemis limited to below 1 MW/cm².
 6. The method of claim 1, wherein thelaser beam is a pulsed laser beam, and wherein the nonlinear crystal ispositioned such that the focus of the pulsed laser beam is outside thenonlinear crystal in the at least one plane perpendicular to the beampropagation direction of the pulsed laser beam with a Rayleigh rangeconfigured such that fundamental optical fluence power density orharmonic optical fluence on or in at least one optical component of thesystem is limited to below 10 J/cm².
 7. The method of claim 1, whereinthe crystal mount assembly is adjustable, and wherein the nonlinearcrystal is positioned such that the focus of the laser beam is outsidethe nonlinear crystal in the at least one plane perpendicular to thebeam propagation direction of the laser beam with a Rayleigh rangeconfigured such that time-averaged fundamental optical power density orharmonic optical power density at the spatial peak of the beam profileon or in at least one optical component in the system is limited tobelow 1 MW/cm².
 8. The method of claim 1, wherein the laser beam is apulsed laser beam, wherein the crystal mount assembly is adjustable, andwherein the nonlinear crystal is positioned such that the focus of apulsed laser beam is outside the nonlinear crystal in the at least oneplane perpendicular to the beam propagation direction of the pulsedlaser beam with a Rayleigh range configured such that fundamentaloptical fluence or harmonic optical fluence on or in at least oneoptical component in the system is limited to below 10 J/cm².
 9. Themethod of claim 1, wherein the focus is at least one of circular,elliptical, or astigmatic.
 10. The method of claim 1, wherein the focusis elliptical, and wherein the focus in a plane parallel to a walk-offis larger than the focus in a plane perpendicular to the walk-off. 11.The method of claim 1, wherein the focus is astigmatic, and wherein thefocus in one plane is inside the nonlinear crystal and the focus inanother plane is outside the nonlinear crystal.
 12. The method of claim1, wherein the focus is astigmatic and elliptical, wherein the focus inone plane is inside the nonlinear crystal and the focus in another planeis outside the nonlinear crystal, and wherein the focus inside thenonlinear crystal has a larger width than the focus outside thenonlinear crystal.
 13. The method of claim 1, further comprisingfastening the crystal mount assembly into one of the mounting features.14. The method of claim 13, wherein the fastening comprises screwing thecrystal mount assembly into one of the mounting features.